Question 1

How does the midpoint rule approximate volume in a double integral?

Accepted Answer

The midpoint rule divides a rectangular region into smaller sub-rectangles of equal size. Within each sub-rectangle, the function is evaluated at the midpoint to determine the approximate height. The volume of each vertical column equals the height at the midpoint multiplied by the sub-rectangle's area. Summing all column volumes gives the total approximate volume over the region.

Question 2

Why is the midpoint rule useful for civil engineering earthwork calculations?

Accepted Answer

In civil engineering, the midpoint rule estimates soil volume between terrain and a planned road level. The road footprint is modeled as a rectangle, and terrain elevation is represented by a continuous height function. By partitioning the region and evaluating height at each midpoint, engineers can approximate excavation or fill requirements without computing exact integrals, making it practical for real-life applications of multiple integrals.

Question 3

What is the relationship between sub-rectangle area and the midpoint rule formula?

Accepted Answer

Each sub-rectangle has an area ΔA = Δx · Δy, where Δx and Δy are the widths in the x and y directions. The volume of soil in each column is calculated by multiplying the height at the midpoint, f(xi*, yj*), by this area. The total approximate volume is the sum of all such products across all sub-rectangles in the partition.

Question 4

How does partitioning the rectangular region improve the midpoint rule approximation?

Accepted Answer

Dividing the rectangular base into m intervals in the x-direction and n intervals in the y-direction creates smaller, more manageable sections. As the region is divided into finer sub-rectangles, the midpoint height becomes a more accurate representative value for each section. This refinement increases the accuracy of the total volume approximation.

Question 5

What role does the continuous height function play in the midpoint rule?

Accepted Answer

The continuous height function f(x,y) represents the terrain elevation above a reference level at any point in the rectangular region. At each sub-rectangle's midpoint (xi*, yj*), the function value f(xi*, yj*) serves as the approximate height for that entire section. This function value, multiplied by the sub-rectangle area, estimates the soil volume in that column.

Question 6

How is the total approximate volume calculated using the midpoint rule?

Accepted Answer

The total approximate volume is obtained by summing the volumes of all vertical columns: the double sum of f(xi*, yj*) · ΔA over all sub-rectangles. Each term represents one column's volume, calculated by multiplying the midpoint height by the sub-rectangle area. Adding these contributions across the entire partitioned region yields the overall soil volume estimate.

Question 7

What determines the accuracy of the midpoint rule approximation?

Accepted Answer

Accuracy depends on the fineness of the partition. As sub-rectangles become smaller, the midpoint height more closely represents the average height across each section. Finer partitions with more sub-rectangles produce better approximations because the height variation within each smaller section is reduced, making the midpoint value more representative of the entire section.

La règle du point médian pour les intégrales doubles

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